Narrow Band Random Fatigue Testing with Amsler Vibrophore Machines

Abstract

A description is given of modifications made to Amsler Vibrophore machines to permit narrow band random fatigue tests to be made. In this mode of working, the machine electromagnet is excited by means of a random generator and a complete control loop is effected by connecting the output of the inductive transducer of the machine dynamometer to the automatic output regulator (compressor circuit) of the random generator. The dynamometer was strain-gauged and calibrated to read r.m.s. load. Under compressor circuit control, the stress peaks in a specimen followed the Rayleigh distribution at least up to four times the r.m.s. value.As in constant amplitude sinusoidal loading, these machines are capable of applying mean stress when working in random loading. Some practically occurring stress spectra may be synthesized by applying sequences consisting of a number of stepped r.m.s. loads, each step being applied for a predetermined time.